Did a secret search for Marco Polo’s islands of gold lead Portuguese
explorers to be the first Europeans to discover Australia?
According to some theories, the Dieppe maps, a series of artful 16th century maps say yes.
Operating in the mid-1500s, the Dieppe mapmakers created elaborate,
hand-made world maps for wealthy patrons and royals.
The French artists
who created the maps were just that, leaving the actual exploration to
others and simply translating more utilitarian nautical charts into
things of beauty.
The surviving maps are beautifully rendered, although
their exact cartographic sources seem to have been lost to time.
This
becomes most problematic in the case of "Java la Grande", a giant
landmass unique to the maps that was drawn between Antarctica and what
we would today consider to be Indonesia.
According to some modern
researchers, this mystery island is actually the first record of
Europeans seeing Australia.

The map has been inverted to represent the modern view, but Java la Grande can be found where Australia would be.

The maps, with their fancy compass roses and detailed illustrations,
were intended to be pieces of art, rather than navigational aids, but
their information had to come from somewhere.
The names and script on
the charts are written out in a mix of French and Portuguese, giving
rise to the theory, which was popularized in Kenneth McIntyre's 1977
book, The Secret Discovery of Australia, that
the mapmakers of Dieppe were getting their view of the world, at least
in part, from Portuguese expeditions.
In particular, one of the maps
that came out of Dieppe, (and is survived by a faithful recreation)
depicts the east coast of the fabled Java la Grande with place names
almost exclusively in Portuguese.
Given the vagaries of the Dieppe map
sources, this has led to the theory that it was the Portuguese who were
the first Europeans to spy the Australian coast.

In addition to the general location of Java la Grande on the maps,
there are certain features that adherents to the theory claim are
unmistakably bits of Australia, such as an inlet that looks like Botany
Bay and the Abrolhos island chain.

Java la Grande was thought to be so big the map was awkwardly extended.

As to what expedition could have seen the coast, it is suggested by
McIntyre that it was a search for Marco Polo’s fabled Isles of Gold that
led to the discovery.
Wealthy Portuguese explorer Cristóvão de Mendonça
is recorded as having been tasked by King Manuel with sailing out in
search of Polo’s treasure islands, but actual record of this voyage has
been lost, if there ever was one.
Manuel was notoriously secretive about
the findings of his exploration teams.
According to popular history,
Australia was first visited by Europeans when Dutch explorer Willem
Janszoon “discovered” the continent in the early 17th century, and later
fully explored by Captain Cook.

On the left, first Portuguese chart designed in Dieppe by Jean Rotz in 1542.

On the right, Australia seen by Dutch in 1628...

While no direct evidence of Portuguese
discovery exists, there have been other findings that seem to support
the theory of their early Australian discovery.
Various ruins, cannons,
and other archeological artifacts have been found on the Australian
continent that believers say point to Portuguese discovery, but the
Dieppe maps remains the prime source of speculation.